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          <h1 class="post-title" itemprop="name headline">Linux too many open files问题调查</h1>
        

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        <h2 id="问题"><a href="#问题" class="headerlink" title="问题"></a>问题</h2><p>今天发现某netty服务总是报too many open files，因为是新上的服务，设备量并不多，不应该啊，T.T</p>
<h2 id="原因"><a href="#原因" class="headerlink" title="原因"></a>原因</h2><p>调查一下发生原因：<br>too many open files，字面意思上看就是打开了过多的文件，不过在linux下files并不只是文件的意思，像socket(Tcp、Udp)连接，开启监听的端口这些都属于file，所以有时候也可以叫做句柄(handle)，这个错误也可以描述为句柄数（file-handles）超出系统限制。 </p>
<p>从上述文字描述可以看出，出现这句提示的原因就是程序打开的Socket连接数量超过系统设定值。</p>
<p>既然是超过了系统的设定值，那么当前用户的设定值是多少呢？</p>
<p>用命令查一下：<br><figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line">[root@localhost ~]# ulimit -a</span><br><span class="line">core file size          (blocks, -c) 0</span><br><span class="line">data seg size           (kbytes, -d) unlimited</span><br><span class="line">scheduling priority             (-e) 0</span><br><span class="line">file size               (blocks, -f) unlimited</span><br><span class="line">pending signals                 (-i) 31203</span><br><span class="line">max locked memory       (kbytes, -l) 64</span><br><span class="line">max memory size         (kbytes, -m) unlimited</span><br><span class="line">open files                      (-n) 1024</span><br><span class="line">pipe size            (512 bytes, -p) 8</span><br><span class="line">POSIX message queues     (bytes, -q) 819200</span><br><span class="line">real-time priority              (-r) 0</span><br><span class="line">stack size              (kbytes, -s) 8192</span><br><span class="line">cpu time               (seconds, -t) unlimited</span><br><span class="line">max user processes              (-u) 31203</span><br><span class="line">virtual memory          (kbytes, -v) unlimited</span><br><span class="line">file locks                      (-x) unlimited</span><br></pre></td></tr></table></figure></p>
<p>或者</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">[root@localhost ~]# ulimit -n</span><br><span class="line">1024</span><br></pre></td></tr></table></figure>
<p>当前用户open files居然才只有1024，查看一下当前系统已开启的文件数</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">[root@localhost ~]# lsof | wc -l</span><br><span class="line">-bash: lsof: command not found</span><br></pre></td></tr></table></figure>
<p>0.0，没有安装lsof，yum安装一下</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br></pre></td><td class="code"><pre><span class="line">[root@localhost ~]# yum install lsof</span><br><span class="line">Loaded plugins: fastestmirror</span><br><span class="line">base                                                     | 3.6 kB     00:00</span><br><span class="line">epel/x86_64/metalink                                     | 6.7 kB     00:00</span><br><span class="line">epel                                                     | 3.2 kB     00:00</span><br><span class="line">extras                                                   | 3.4 kB     00:00</span><br><span class="line">updates                                                  | 3.4 kB     00:00</span><br><span class="line">(1/3): epel/x86_64/updateinfo                              | 932 kB   00:00</span><br><span class="line">(2/3): updates/7/x86_64/primary_db                         | 2.0 MB   00:01</span><br><span class="line">(3/3): epel/x86_64/primary                                 | 3.5 MB   00:01</span><br><span class="line">Loading mirror speeds from cached hostfile</span><br><span class="line"> * base: mirrors.aliyun.com</span><br><span class="line"> * epel: mirrors.aliyun.com</span><br><span class="line"> * extras: mirrors.aliyun.com</span><br><span class="line"> * updates: mirrors.aliyun.com</span><br><span class="line">epel                                                                12585/12585</span><br><span class="line">Resolving Dependencies</span><br><span class="line">--&gt; Running transaction check</span><br><span class="line">---&gt; Package lsof.x86_64 0:4.87-5.el7 will be installed</span><br><span class="line">--&gt; Finished Dependency Resolution</span><br><span class="line"></span><br><span class="line">Dependencies Resolved</span><br><span class="line"></span><br><span class="line">================================================================================</span><br><span class="line"> Package         Arch              Version                Repository       Size</span><br><span class="line">================================================================================</span><br><span class="line">Installing:</span><br><span class="line"> lsof            x86_64            4.87-5.el7             base            331 k</span><br><span class="line"></span><br><span class="line">Transaction Summary</span><br><span class="line">================================================================================</span><br><span class="line">Install  1 Package</span><br><span class="line"></span><br><span class="line">Total download size: 331 k</span><br><span class="line">Installed size: 927 k</span><br><span class="line">Is this ok [y/d/N]: y</span><br><span class="line">Downloading packages:</span><br><span class="line">lsof-4.87-5.el7.x86_64.rpm                                 | 331 kB   00:00</span><br><span class="line">Running transaction check</span><br><span class="line">Running transaction test</span><br><span class="line">Transaction test succeeded</span><br><span class="line">Running transaction</span><br><span class="line">  Installing : lsof-4.87-5.el7.x86_64                                       1/1</span><br><span class="line">  Verifying  : lsof-4.87-5.el7.x86_64                                       1/1</span><br><span class="line"></span><br><span class="line">Installed:</span><br><span class="line">  lsof.x86_64 0:4.87-5.el7</span><br><span class="line"></span><br><span class="line">Complete!</span><br></pre></td></tr></table></figure>
<p>再查看该进程已打开的文件数</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"># wc -l 统计输出信息的行数</span><br><span class="line"># 因为lsof -p pid已经过滤到该进程已打开的文件数，一行信息对应一个文件，所以也就是文件的个数。</span><br><span class="line">[root@localhost ~]# lsof -p pid | wc -l</span><br><span class="line">2858</span><br></pre></td></tr></table></figure>
<h2 id="解决方案"><a href="#解决方案" class="headerlink" title="解决方案"></a>解决方案</h2><p>临时提高一下open files数值</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line">ulimit -n 65535</span><br></pre></td></tr></table></figure>
<p>再查看发现open files已更新，但是这种一重启就会还原</p>
<p>永久更改open files的方法</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">vim /etc/security/limits.conf  </span><br><span class="line"># insert last line</span><br><span class="line"># * 标识所有用户，也可单独设置某一用户，如test soft nofile 65535</span><br><span class="line">* soft nofile 65535  </span><br><span class="line">* hard nofile 65535</span><br></pre></td></tr></table></figure>
<h2 id="深度调查"><a href="#深度调查" class="headerlink" title="深度调查"></a>深度调查</h2><p>使用命令打印程序open files详细日志，进行问题排查。</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br></pre></td><td class="code"><pre><span class="line">[root@localhost /]# lsof -p pid &gt; openfiles.log</span><br><span class="line">[root@localhost /]# tail -f ./openfiles.log</span><br><span class="line"></span><br><span class="line">java    5275 root 2864u     IPv6           23540972       0t0       TCP localhost.localdomain:45214-&gt;192.168.0.199:19092 (ESTABLISHED)</span><br><span class="line">java    5275 root 2865u     IPv6           23540973       0t0       TCP localhost.localdomain:45216-&gt;192.168.0.199:19092 (ESTABLISHED)</span><br><span class="line">java    5275 root 2866u  a_inode                0,9         0      5815 [eventpoll]</span><br><span class="line">java    5275 root 2867u     IPv6           23541135       0t0       TCP localhost.localdomain:45398-&gt;192.168.0.199:19092 (ESTABLISHED)</span><br><span class="line">java    5275 root 2868u     IPv6           23541136       0t0       TCP localhost.localdomain:45401-&gt;192.168.0.199:19092 (ESTABLISHED)</span><br><span class="line">java    5275 root 2869r     FIFO                0,8       0t0  23541164 pipe</span><br><span class="line">java    5275 root 2870w     FIFO                0,8       0t0  23541164 pipe</span><br><span class="line">java    5275 root 2871u  a_inode                0,9         0      5815 [eventpoll]</span><br><span class="line">java    5275 root 2872u     IPv6           23541165       0t0       TCP localhost.localdomain:45433-&gt;192.168.0.199:19092 (ESTABLISHED)</span><br><span class="line">java    5275 root 2873u     IPv6           23541166       0t0       TCP localhost.localdomain:45434-&gt;192.168.0.199:19092 (ESTABLISHED)</span><br><span class="line">....</span><br></pre></td></tr></table></figure>
<p>发现有很多连接处在ESTABLISHED（通讯中）状态，应该是硬件频繁连接，netty持有的channel并没有被释放。</p>
<h2 id="程序解决方案"><a href="#程序解决方案" class="headerlink" title="程序解决方案"></a>程序解决方案</h2><p>服务添加idle处理，硬件心跳中断一段时间后自动将channel关闭。</p>
<h2 id="TCP端口状态说明"><a href="#TCP端口状态说明" class="headerlink" title="TCP端口状态说明"></a>TCP端口状态说明</h2><h3 id="1、LISTENING状态"><a href="#1、LISTENING状态" class="headerlink" title="1、LISTENING状态"></a>1、LISTENING状态</h3><p>FTP服务启动后首先处于侦听（LISTENING）状态。</p>
<h3 id="2、ESTABLISHED状态"><a href="#2、ESTABLISHED状态" class="headerlink" title="2、ESTABLISHED状态"></a>2、ESTABLISHED状态</h3><p>ESTABLISHED的意思是建立连接。表示两台机器正在通信。</p>
<h3 id="3、CLOSE-WAIT"><a href="#3、CLOSE-WAIT" class="headerlink" title="3、CLOSE_WAIT"></a>3、CLOSE_WAIT</h3><p>对方主动关闭连接或者网络异常导致连接中断，这时我方的状态会变成CLOSE_WAIT 此时我方要调用close()来使得连接正确关闭。</p>
<h3 id="4、TIME-WAIT"><a href="#4、TIME-WAIT" class="headerlink" title="4、TIME_WAIT"></a>4、TIME_WAIT</h3><p>我方主动调用close()断开连接，收到对方确认后状态变为TIME_WAIT。TCP协议规定TIME_WAIT状态会一直持续2MSL(即两倍的分段最大生存期)，以此来确保旧的连接状态不会对新连接产生影响。处于TIME_WAIT状态的连接占用的资源不会被内核释放，所以作为服务器，在可能的情况下，尽量不要主动断开连接，以减少TIME_WAIT状态造成的资源浪费。目前有一种避免TIME_WAIT资源浪费的方法，就是关闭socket的LINGER选项。但这种做法是TCP协议不推荐使用的，在某些情况下这个操作可能会带来错误。</p>
<h3 id="5、SYN-SENT状态"><a href="#5、SYN-SENT状态" class="headerlink" title="5、SYN_SENT状态"></a>5、SYN_SENT状态</h3><p>SYN_SENT状态表示请求连接，当你要访问其它的计算机的服务时首先要发个同步信号给该端口，此时状态为SYN_SENT，如果连接成功了就变为 ESTABLISHED，此时SYN_SENT状态非常短暂。但如果发现SYN_SENT非常多且在向不同的机器发出，那你的机器可能中了冲击波或震荡波之类的病毒了。这类病毒为了感染别的计算机，它就要扫描别的计算机，在扫描的过程中对每个要扫描的计算机都要发出了同步请求，这也是出现许多SYN_SENT的原因。</p>
<p>根据TCP协议定义的3次握手断开连接规定,发起Socket主动关闭的一方Socket将进入TIME_WAIT状态，TIME_WAIT状态将持续2个MSL(Max Segment Lifetime)，在Windows下默认为4分钟,即240秒，TIME_WAIT状态下的socket不能被回收使用。具体现象是对于一个处理大量短连接的服务器，如果是由服务器主动关闭客户端的连接,将导致服务器端存在大量的处于TIME_WAIT状态的socket，甚至比处于Established状态下的socket多的多，严重影响服务器的处理能力,甚至耗尽可用的socket，停止服务。TIME_WAIT是TCP协议用以保证被重新分配的socket不会受到之前残留的延迟重发报文影响的机制，是必要的逻辑保证。</p>

      
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